Hot gases treated in certain industrial processes contain components that have a tendency to stick on, for example, heat exchange surfaces. Sticky compounds may also be generated as a result of cooling. This complicates the recovery of heat from the gases or cooling the gas.
Problems also appear in the gasification processes because of the substances mat stick on heat exchange surfaces. Gasification or combustion of solid carbonaceous material in a circulating fluidized bed reactor, in which such a high gas flow velocity is maintained that a considerable portion of the solid particles is entrained with gas from the reaction chamber and after particle separation mainly returned to the fluidized bed, has been noted to have many advantages as compared to conventional gasification or combustion methods.
When gasifying carbonaceous fuels, such as biofuels or waste-derived fuels, generally, air and/or oxygen, as well as steam, are supplied, to the gasification reactor, whereby an object is to generate product gas, the main components of which are carbon monoxide CO, hydrogen H2, and hydrocarbons CxHy. Ash particles and residual carbon are usually entrained with the product gas exiling from the gasification reactor. Depending on the concept, they must possibly be separated by a particle separator, for example, by a filter, prior to further use of the product gas. Generally, the aim is to optimize the efficiency of the gasification system in such a way that the coal conversion level of the fuel is as high as possible, in other words, the content of the residual carbon in the ash removed from the equipment is as low as possible.
Especially, with the gasification gases derived from biofuels, heat recovery and also, possibly, further use of the gas are substantially complicated by components that are contained in the biofuels and have a tendency to stick on, for example, the heat exchange surfaces. Sticky compounds may also be generated as a result of cooling.
The product gas exiting from the gasification reactor also generally contains ash particles, which need to be removed, for example, by means of a particle filter prior to further use of the product gas. Since the particle filters that filter the gas at a high temperature are expensive and are prone to being damaged, the product gas is generally cooled prior to the filtering. Especially, when gasifying waste materials and biomass, considerable amounts of tar compounds can be generated. Here, tar compounds refer to compounds or components that are gaseous at the gasification temperature, but are condensed at lower temperatures to droplets, which stick easily, and further, even to solid particles, which can build up, for example, on heal exchange surfaces of the gas cooler or cause filter deposits that are difficult to be removed. Thus, tar compounds, for example, reduce the heat exchange efficiency of the heat exchange surfaces, weakening the operation of the equipment and clogging the filtering elements of the filter, thereby, increasing die pressure loss.
The amount of tar compounds can be diminished by means of thermal cracking. The tar compounds are then decomposed by thermal cracking, and the amount of tar compounds in the final product gas diminishes. The thermal cracking of the product gas is performed by raising the gas temperature, after gasification, high enough, whereby, die generated tars are decomposed to simpler compounds. The simplest way to do this is to introduce either oxygen or air to the product gas. A portion of the combustible components of the gas thereby burns and the temperature rises. The temperature required for cracking of tar compounds is about 1000° C. to about 1200° C. The product gas consumed for combustion is compensated for by compounds generated in thermal cracking.
Japanese Patent Publication No. 11-043681 discloses gasification of biofuels in a fluidized bed reactor. The product gas from a fluidized bed reactor is guided to an oxidizing oven operating at a temperature higher than that of the fluidized bed reactor, in which oven, secondary gasification takes place. The temperature in the oxidizing oven is about 1200° C. to about 1600° C., whereby, for example, tar compounds decompose. The lower portion of the oxidizing oven is provided with a cooling portion, in which gas and the formed melt material are cooled by conducting them to water. The quick water cooling solidies the melt material, the thus granulated material is removed from the cooler, and the gas is guided to further treatment.
U.S. Patent Application Publication No. 2007/0175095 discloses a biomass gasification system, in which the product gas from the actual gasification stage is conducted to a downstream reforming unit, in which the tar components of the product gas are decomposed by thermal cracking. Oxygen is supplied to the reforming unit, whereby the fuel oxidizes, which increases the temperature to a level required by the thermal cracking. This causes cracking of tar compounds. Gas is cooled after die reforming unit and conducted to be used. Here, the melt material from the gasification stage is led to act as fuel in a separate heater that provides heat to the gasification. In the method disclosed in this publication, the question of the treatment of melt components generated in the actual thermal cracking remains completely open. Thus, the solution is especially prone to clogging of the heat surfaces downstream of the reforming unit.